Burn, Magnet, Burn

Mountaineers who tremble at the thought of an avalanche have some
company in the physics lab. A kind of avalanche that occurs in
magnetic crystals is just as destructive to physicists' experiments
as is crashing snow to an unsuspecting skier. In so-called magnetic
avalanches, the molecule-sized magnets that make up a magnetic
crystal suddenly flip their spins. These uncontrolled events,
discovered in the early 1990s, were catastrophic to experiments, but
their cause and propagation remained a puzzle. The solution, it
turns out, may be of more relevance to fire fighters than to ski patrols.

Myriam P. Sarachik and graduate student Yoko Suzuki, at the City
College of the City University of New York (CUNY), set up an
experiment to track magnetic avalanches. The results led their
collaborator, Eugene M. Chudnovsky of CUNY's Lehman College, to
recognize an unexpected phenomenon: The magnetic avalanche
progressed through the crystal exactly like a flame front moving
through a chemically burnable substance, such as a piece of paper.
As the group reported in the September 30, 2005, issue of
Physical Review Letters, these annoying events may turn
out to be a boon for engineers studying
deflagration—the movement of a flame through a material.

At the center of this discovery are crystals only about a millimeter
long, grown by Sarachik's collaborators at the University of Florida
in Gainesville. These crystals are made of manganese-12 acetate,
widely used to study quantum tunneling and a contender for use in
magnetic memory storage and quantum computing. Using
micrometer-sized sensors created at the Weizmann Institute of
Science in Israel, Suzuki put together an electronic measurement
system fast enough to take readings at 11 points along the length of
the crystal during the avalanche. Prior to this, investigators had
looked only at what happened to the magnetic field of the entire
crystal during an avalanche.

Suzuki submerged the sensor-laden crystals in liquid helium-3 at
temperatures below one kelvin—very close to absolute zero. She
placed the apparatus in a magnetic field and gradually varied the
field strength until an avalanche occurred. She found that there was
a threshold energy, a field strength above which an avalanche would
abruptly commence. She also found that the avalanches always seemed
to start from an edge and progress in a wave, reversing the spins of
the crystal's molecular magnets in an abrupt front traveling down
the length of the crystal. The speed of the avalanche was constant
but depended on the strength of the external magnetic field, varying
from one to 15 meters per second.

Chudnovsky saw in the results an analogy to chemical burning. The
moving band of flipping molecular magnetic spins in the crystal is
equivalent to a flame front progressing through a piece of paper,
for instance, and the area behind the front, where the molecular
magnet spins had flipped to align with the external field, is like
the ash left behind a fire. The material ahead of the magnetic
avalanche is analogous to unburned material. "We took formulas
for how a flame propagates in a chemical substance, which actually
are a hundred years old, and they work perfectly well in describing
this phenomenon," Chudnovsky says. "That's how we were
convinced that this was a new phenomenon, magnetic deflagration."

The group still doesn't entirely understand what conditions set off
the avalanche. "It's a probabilistic event," explains
Sarachik. "We sweep the magnetic field and then at some field
it goes boom, and you can't predict it." But in the same way
that a fire requires energy to sustain itself, the
"ignition" of a magnetic avalanche also seems to require a
magnetic field of a certain strength; near the threshold energy it
peters out partway through the crystal. "It's like a fire that
goes out before it finishes its fuel," says Sarachik. However,
once there is successful ignition, when the molecular magnets's
spins in one area flip over to line up with the external magnetic
field, they release energy that helps the next section to flip as
well, propagating the process.

Sarachik says scientists who study deflagration might use this
set-up as a more controllable way to look at flame fronts. In
addition, she notes, the crystals are reusable. In experiments
following flame fronts along paper, "you have to use a
different sample every time, and one piece of paper may be different
from the next. In these crystals, the ‘ash' behind the
propagating front has spins pointing in the direction of the field,
but there's nothing destructive about that. So you can repeat the
experiment over and over, without any doubt whether it's the same
sample or not." Sarachik and her colleagues are now planning to
see whether there is a magnetic equivalent to detonation as well.

The cognitive leap that Chudnovsky made between magnetism and
burning is also a testament to a well-rounded education.
"Deflagration is mostly studied by engineers who work on rocket
propellants and gasoline engines," he said. "These days I
think very few physicists know what deflagration is. But to get my
Ph.D. in Russia, I was required to study all subjects in physics. So
I was lucky that at some time in my life I studied this material."